rha030-workbook11-student-3.0-0

Workbook 11. Supplements

Red Hat, Inc.

Workbook 11. Supplementsby Red Hat, Inc.Copyright © 2003-2005 Red Hat, Inc.

Revision HistoryRevision rha030-2.0-2003_11_12-en 2003-11-12First RevisionRevision rha030-3.0-0-en-2005-08-17T07:23:17-0400 2005-08-17First Revision

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Published 2005-08-17

Table of Contents1. Advanced Shell Scripting ...................................................................................................................... 4

Discussion .......................................................................................................................................... 4Shell Scripting .......................................................................................................................... 4Branches: if ... then ... [else ...] fi ..................................................................4Loops: for ... in ... do ... done .............................................................................. 8

Examples............................................................................................................................................ 9Example 1. A Script for "Packing" Directories ........................................................................ 9

Online Exercises............................................................................................................................... 11Specification ........................................................................................................................... 11Deliverables ............................................................................................................................ 11

Questions.......................................................................................................................................... 112. Character Encoding and Internationalization.................................................................................. 16

Files .................................................................................................................................................. 16What are Files? ....................................................................................................................... 16What is a Byte?....................................................................................................................... 16Data Encoding ........................................................................................................................ 17

Text Encoding .................................................................................................................................. 17ASCII...................................................................................................................................... 17ISO 8859 and Other Character Sets........................................................................................ 19Unicode (UCS) ....................................................................................................................... 19Unicode Transformation Format (UTF-8).............................................................................. 20Text Encoding and the Open Source Community ..................................................................21

Internationalization (i18n)................................................................................................................ 21The LANG environment variable ............................................................................................. 22Do I Really Have to Know All of This? ................................................................................. 23

3. The RPM Package Manager............................................................................................................... 25Discussion ........................................................................................................................................ 25

RPM: The Red Hat Package Manager.................................................................................... 25RPM Components .................................................................................................................. 25Querying the RPM database................................................................................................... 26

Online Exercises............................................................................................................................... 30Specification ........................................................................................................................... 31Deliverables ............................................................................................................................ 31

Questions.......................................................................................................................................... 32

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Chapter 1. Advanced Shell Scripting

Key Concepts• Linux uses a general scripting mechanism, where executable text script files can be executed by an

interpreter specified on the initial line.

• Within a bash script, any arguments provided when the script was invoked, are available as positionalparameters (i.e, the variables $1, $2, ...).

• The read builtin command can be used to read input from the keyboard ("standard in").

• The bash shell uses a if ... then ... [else ...] fi syntax to implement conditionalbranches.

• The test command is often used as the conditional command in if ... then branches.

• The bash shell uses a for ... in ... do ... done syntax to implement loops.

Discussion

Shell ScriptingEarlier chapters of this workbook discussed the creation of simple shell scripts. These scripts did littlemore than execute a series of commands, optionally accepting user input to define variables.

However, shell scripts are capable of much, much more of this. This chapter will add some valuable toolsto your arsenal, allowing your scripts to make basic if/then/else decisions and loop a set of actionsindefinitely.

Branches: if ... then ... [else ...] fi

Bash syntax for branchesIn programming, branches allow programs to choose between one of two (or more) alternate executionpaths. The bash shell, like most programming languages, uses the word if to signify a branch. Moreformally, bash uses the following syntax.

if conditionthencommand(s) Ê

...fi

or

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if conditionthencommand(s) Ê

...elsecommand(s) Ë

...fi

Ê The commands in these stanzas are executed if the condition "succeeds".

Ë The commands in this stanza are executed if the condition "fails".

When using this syntax, carriage returns are important (i.e., the if and then must occur on separatelines), but indentations are not.

What does bash expect as a condition? Unlike most programming languages, bash has no internalsyntax for making comparisons (such as $A == apple, or $B > 25). Instead, bash focuses on what shellswere designed to do: run commands. Any command can be used for the condition. The bash shell willexecute the command, and examine its return value. If the command "succeeds" (returns a return value of0), the the first stanza of commands is executed. If the command fails (returns a return value not equal tozero), the second stanza of commands is executed (if any).

The following modification to elvis’s script shut serves as an example.

[elvis@station elvis]$ lsexample.sh shut[elvis@station elvis]$ cat shut#!/bin/bash# the first argument should be the name of the file to shut.

if ls $1then

chmod 600 $1else

echo "The file $1 does not exist."fi[elvis@station elvis]$ ./shut example.shexample.sh[elvis@station elvis]$ ./shut fools: foo: No such file or directoryThe file foo does not exist.

In the first case, the ls command "succeeds" ((because the file example.sh exists, the return value fromthe ls command is 0). As a result, the first stanza of the if ... then ... else ... fi clause isexecuted. In the second case, the file foo does not exist, so the second (else) stanza of the clause wasexecuted.

The test CommandThe previous example is admittedly awkward. The ls command is being forced to perform a task forwhich it wasn’t designed: testing for the existence of a file. While it can serve this function, the messages

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it prints are distracting. If the specified file exists, its name is printed to the screen. If the file does notexist, then an error message is printed. If only there was a command which would check for the existenceof a file, and return the appropriate return code, without emitting any other messages...

There is. The command is called test. The test command was designed for exactly this purpose: to be asthe conditional command in bash if ... then statements. More specifically, the test command isdesigned to compare strings, integers, and file attributes. It never generates output, but insteadcommunicates using its return value. The test command returns 0 if the expression it evaluates is true,and a non zero value otherwise.

Most of test’s arguments look similar to command line switches. For example, -e tests for the existenceof a file. The above script could be improved by replacing ls with test -e.

[elvis@station elvis]$ cat shut#!/bin/bash# the first argument should be the name of the file to shut.

if test -e $1then

chmod 600 $1else

echo "The file $1 does not exist."fi[elvis@station elvis]$ ./shut example.sh[elvis@station elvis]$ ./shut fooThe file foo does not exist.

Notice that the test command tests for the existence of the file, but does not generate any messages todistract the user.

The following table lists some of the more commonly switches for testing file attributes.

Table 1-1. test Expressions for Examining File Attributes

Expression Condition-d FILE FILE exists, and is a directory.-e FILE FILE exists-f FILE FILE exists, and is a regular file.-r FILE FILE exists, and is readable.-w FILE FILE exists, and is writable.-x FILE FILE exists, and is executable.FILE1 -nt FILE2 FILE1 is newer than FILE2.

Table 1-2. test Expressions for Comparing Strings

Expression Condition[-n] STRING the length of STRING is greater than zero.-z STRING the length of STRING is zero.STRING1 = STRING2 STRING1 and STRING2 are equal.

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of U.S. and international copyrights. No part of this publication may be photocopied, duplicated, stored in a retrieval system, or otherwise duplicated whether in electronic or print

format without prior written consent of Red Hat, Inc. If you believe Red Hat course materials are being used, copied, or otherwise improperly distributed please email

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Expression ConditionSTRING1 != STRING2 STRING1 and STRING2 are not equal.

Lastly, the following table lists expressions that allow the test command to use compound logic.

Table 1-3. Logic Expressions for the test Command

Expression ConditionEXPRESSION1 -a EXPRESSION2 Both EXPRESSION1 and EXPRESSION2 are true.EXPRESSION1 -o EXPRESSION2 Either EXPRESSION1 or EXPRESSION2 is true.! EXPRESSION EXPRESSION is false.

These tables are meant to provide the student with a usable working set of expressions. For a completelisting, consult the test(1) man page.

Alternate expression for test: [ expression ]The test command is so commonly used in bash scripting, that a shorter syntax has been developed. Thefollowing two expressions are equivalent.

test expression

[ expression ]

As an example, elvis’s shut script could be rewritten as the following.


if [ -e $1 ] Ê

thenchmod 600 $1

elseecho "The file $1 does not exist."

fi

Ê Notice that the test command has been replaced with the alternate [ ... ] syntax.

When using the alternate syntax, care must be taken to include a space after the opening bracket, andbefore the closing bracket. 1 For example, the following two constructions of the test command arewrong.

[-e foo.sh ][ -e foo.sh]

The next construction is really wrong.

[-e foo.sh]

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Loops: for ... in ... do ... done

Bash syntax for loopsLoops are perhaps the most useful programming structure for automating mundane tasks. Loops allow aseries of commands to be repeated, usually with slight variations in each iteration. Usually, thesevariations are implemented using a variable referred to as an iterator. For each iteration of the loop, thevariable takes own a different value. For example, elvis could use the following script to affirm hisaffection for his collection of household pets.

[elvis@station elvis]$ cat nice#!/bin/bash

for PET in kitty doggy gerbil newtdo

echo "nice $PET."done[elvis@station elvis]$ ./nicenice kitty.nice doggy.nice gerbil.nice newt.

In this script, the shell variable PET is used as the iterator. With each iteration of the loop, the variabletakes on a different value.

More formally, for ... in ... do ... done loops in bash use the following syntax.

for iterator in listdocommand(s)...

done

For each repetition of the loop, the variable iterator will evaluate to the individual words listed in theexpression list.

For a more practical example, we revisit elvis’s script shut. The user elvis would now like to modify hisscript, so that he can specify multiple files on the command line. To implement this change, heessentially takes his previous script, and wraps it inside a for ... in .. do ... done loop. Ratherthan using the first positional parameter ($1) directly, elvis uses an iterator to iterate through allarguments supplied on the command line.


for FILE in $*do

if [ -e $FILE ]then

chmod 600 $FILEelse

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echo "The file $FILE does not exist."fi

done

In the following, elvis uses the script to modify the permissions on the files example.sh and nice.

[elvis@station elvis]$ ls -ltotal 12-rwxr-xr-x 1 elvis elvis 212 Sep 3 10:56 example.sh-rwxr-xr-x 1 elvis elvis 77 Sep 4 12:16 nice-rwxrwxr-x 1 elvis elvis 188 Sep 4 12:31 shut[elvis@station elvis]$ ./shut example.sh biz nice bazThe file biz does not exist.The file baz does not exist.[elvis@station elvis]$ ls -ltotal 12-rw------- 1 elvis elvis 212 Sep 3 10:56 example.sh-rw------- 1 elvis elvis 77 Sep 4 12:16 nice-rwxrwxr-x 1 elvis elvis 188 Sep 4 12:31 shut

Notice the use of the $* variable to generate the list. The following table suggests other commonly usedtricks of the trade.

Table 1-4. Common Techniques for Generating Iteration Lists

When you use... ... $i iterates through...for i in $* the script’s command line argumentsfor i in /etc/*.conf the file’s matched by the glob /etc/*.conf

for i in $(command) the words returned by the command command.

Examples

Example 1. A Script for "Packing" DirectoriesThe user elvis finds that he is often "tarring up" (archiving) directories he is not actively using. Hedecides to create a script called pack which will help him archive directories more quickly.

The pack script expects one or more directories to be listed as arguments. For each directory, the scriptwill create an archive named after the directory, with the extension .tgz appended. If, and only if, thecreation of the archive is successful, the script will then remove the original directory.

As elvis thinks through the directories that users could specify, he realizes that the directories . and ..could cause problems (why?), so he adds an exclusion for them.

[elvis@station elvis]$ cat pack#!/bin/bash


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for DIR in $*; do Ê

if [ -d $DIR ] Ë

thenif [ "$DIR" == "." -o "$DIR" == ".." ] Ì

thenecho "skipping directory $DIR"

elsetar cvzf $DIR.tgz $DIR && rm -fr $DIR Í

fielse

echo "skipping non directory $DIR"fi

done

Ê The script loops through all of the provided command line arguments.

Ë The script confirms that the argument exists, and that it refers to a directory.

Ì The script here checks that the user has not specified the directory . or ... In practice, there are stillsome directory names that can cause problems. Can you think of any?

Í Finally, here is the line that does the hard work. Notice that the original directory is removed only ifthe tar command succeeds.

He tries out his script on two test directories.

[elvis@station elvis]$ mkdir test{1,2}[elvis@station elvis]$ touch test{1,2}/{one,two,three,four}[elvis@station elvis]$ ls -R.:pack test1 test2

./test1:four one three two

./test2:four one three two[elvis@station elvis]$ ./pack test1 test2test1/test1/onetest1/twotest1/threetest1/fourtest2/test2/onetest2/twotest2/threetest2/four[elvis@station elvis]$ ls -R.:pack test1.tgz test2.tgz

rha030-3.0-0-en-2005-08-17T07:23:17-0400


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The test directories have been "pack"ed.

Online Exercises

Lab ExerciseObjective: Use shell scripting to automate the rotation of images.

Estimated Time: 30 mins.

SpecificationCreate a script called rotate_cw, which can be used to rotate images 90 degrees. In order to performthe rotation, you should use the convert command (examine the convert(1) man page, paying particularattention to the rotate option). The following provides an example of using the convert command torotate an image.

[elvis@station elvis]$ cp /usr/share/pixmaps/redhat-main-menu.png .[elvis@station elvis]$ convert -rotate 90 redhat-main-menu.png /tmp/redhat-main-menu.png

The file /tmp/redhat-main-menu.png is the same image as redhat-main-menu.png, rotated 90degrees.

The script should expect as arguments multiple filenames of the images to be rotated. You may assumethat the filenames will not contain directory components (i.e., they will refer to files in the currentdirectory.) The script should generate a new file of the rotate image, and if the new image is successfullygenerated, replace the original image with the rotated image (giving the appearance of rotating theimages "in place").

Leave the script in your home directory, and ensure that it has executable permissions.

Deliverables

1. An executable bash script called ~/rotate_cw, which will rotate images in the local directory whosefilenames (without directory components) are passed as arguments.

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Questions

1. In order to execute a script, what permission(s) must a user have?

( ) a. read permission

( ) b. write permission

( ) c. execute permission

( ) d. A and C

( ) e. All of the above

2. In order to write an executable bash script, what must the first word of the first line look like?

( ) a. ##bash( ) b. #!bash( ) c. !!/bin/bash( ) d. crunch-bang /bin/bash

( ) e. None of the above

3. When using the Linux scripting mechanism, what may be used as an interpreter?

( ) a. Any executable file within the /usr/bin/ directory

( ) b. Any executable file

( ) c. Only executable files which are listed in the file /etc/interpreters

( ) d. Only executables that ignore lines beginning with #.

( ) e. Only files that meet conditions C and D

4. Which of the following are mechanisms for passing information into shell scripts?

( ) a. Invoking the script with command line arguments.

( ) b. Configuring environment variables before invoking the script.

( ) c. Designing the script to read input from the keyboard (standard in).

( ) d. A and B

( ) e. All of the above

[elvis@station elvis]$ cat script#!/bin/bash

for i in $*do

if [ -r $i -a -f $i ] Ê

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thengzip $i

elseecho "cannot compress $i"

donefi[elvis@station elvis]$ ./script rotate_cw./script: line 10: syntax error near unexpected token ‘done’./script: line 10: ‘ done’

5. Which of the following lines could replace the line labeled 1 above, with no effect on script execution?

( ) a. if test -r $i -a -f $i

( ) b. test -r $i -a -f $i

( ) c. if [ -r $i -o -f $i ]

( ) d. if [ -e $i ]


6. What syntax error exists in the script?

( ) a. The words for and do must occur on the same line.

( ) b. There must be no spaces between [ and -r on the line starting if.

( ) c. The gzip command must be specified using an absolute reference.

( ) d. The last line contains the misspelled word fi.

( ) e. The last two lines (containing done and fi) need to be transposed.

7. What does the variable i iterate through (assuming the syntax error mentioned above is fixed).

( ) a. All (non-hidden) files in the local directory

( ) b. All files in the local directory

( ) c. All files which were previously defined in the environment variable named *.

( ) d. All of the command line arguments provided when the script was invoked.


The following text is found in the file /etc/bashrc.

if [ "‘id -gn‘" = "‘id -un‘" -a ‘id -u‘ -gt 99 ]; thenumask 002

elseumask 022

fi

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8. Which of the following best describes the execution of this text?

( ) a. If the current user is a member of more than one group, then set the current shell’s umask to 002,otherwise set it to 022.

( ) b. If the current user’s group name is the same as the user’s username, or the user has a userid greater than99, set the shell’s umask to 002, otherwise set it to 022.

( ) c. If the user is a member of more than 99 groups, set the shell’s umask to 002, otherwise set it to 022.

( ) d. If the current user is the root user, set current shell’s umask to 002, otherwise set it to 022.

( ) e. If the current user’s group name is the same as the user’s username, and the user has a userid greater than99, set the shell’s umask to 002, otherwise set it to 022.

The following text is found in the file /etc/profile.

for i in /etc/profile.d/*.sh ; doif [ -r "$i" ]; then

. $ifi

done

9. Which of the following best describes the execution of this text?

( ) a. For every file in the /etc/profile.d directory that ends .sh, if it is readable, source it.

( ) b. For every file in the /etc/profile.d directory that ends .sh, if it is readable, execute it.

( ) c. For every file in the /etc/profile.d directory that ends .sh, if it is executable, execute it.

( ) d. For every file in the /etc/profile.d directory that ends .sh, if it is not a directory, source it.

( ) e. None of the above.

10. What innocent action could a system administrator make which cause an error when this section of script isexecuted?

( ) a. The administrator could place a file that does not end with .sh in the /etc/profile.d directory.

( ) b. The administrator could place a file that does not have executable permissions in the /etc/profile.ddirectory.

( ) c. The administrator could place a file that does not have read permissions in the /etc/profile.d directory.

( ) d. The administrator could place a file that does not have write permissions in the /etc/profile.ddirectory.

( ) e. The administrator could place a file titled source me.sh in the /etc/profile.d directory.


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Notes1. In order to explore why this is the case, note that there is actually a file called /usr/bin/[. What

does this imply?


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Chapter 2. Character Encoding andInternationalization

Key Concepts• When storing text, computers transform characters into a numeric representation. This process is

referred to as encoding the text.

• In order to accommodate the demands of a variety of languages, several different encoding techniqueshave been developed. These techniques are represented by a variety of character sets.

• The most sophisticated encoding technique is known as the Universal Character Set (UCS), orUnicode.

• The default encoding technique in Red Hat Enterprise Linux is referred to as UTF-8, which allows theflexibility of Unicode but retains ASCII compatibility.

• The LANG environment variable is used to specify a user’s preferred language and character encoding.

Files

What are Files?Linux, like most operating systems, stores information that needs to be preserved outside of the contextof any individual process in files. (In this context, and for most of this Workbook, the term file is meant inthe sense of regular file). Linux (and Unix) files store information using a simple model: information isstored as a single, ordered array of bytes, starting from at first and ending at the last. The number of bytesin the array is the length of the file. 1

What type of information is stored in files? Here are but a few examples.

• The characters that compose the book report you want to store until you can come back and finish ittomorrow are stored in a file called (say) ~/bookreport.txt.

• The individual colors that make up the picture you took with your digital camera are stored in the file(say) /mnt/camera/dcim/100nikon/dscn1203.jpg.

• The characters which define the usernames of users on a Linux system (and their home directories,etc.) are stored in the file /etc/passwd.

• The specific instructions which tell an x86 compatible CPU how to use the Linux kernel to list the filesin a given directory are stored in the file /bin/ls.

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Chapter 2. Character Encoding and Internationalization

What is a Byte?At the lowest level, computers can only answer one type of question: is it on or off? What is it? Whendealing with disks, it is a magnetic domain which is oriented up or down. When dealing with memorychips, it is a transistor which either has current or doesn’t. Both of these are too difficult to mentallypicture, so we will speak in terms of light switches that can either be on or off. To your computer, thecontents of your file is reduced to what can be thought of as an array of (perhaps millions of) lightswitches. Each light switch can be used to store one bit of information (is it on, or is it off).

Using a single light switch, you cannot store much information. To be more useful, an early conventionwas established: group the light switches into bunches of 8. Each series of 8 light switches (or magneticdomains, or transistors, ...) is a byte. More formally, a byte consists of 8 bits. Each permutation of ons andoffs for a group of 8 switches can be assigned a number. All switches off, we’ll assign 0. Only the firstswitch on, we’ll assign 1; only the second switch on, 2; the first and second switch on, 3; and so on. Howmany numbers will it take to label each possible permutation for 8 light switches? A mathematician willquickly tell you the answer is 2^8, or 256. After grouping the light switches into groups of eight, yourcomputer views the contents of your file as an array of bytes, each with a value ranging from 0 to 255.

Data EncodingIn order to store information as a series of bytes, the information must be somehow converted into aseries of values ranging from 0 to 255. Converting information into such a format is called data encoding.What’s the best way to do it? There is no single best way that works for all situations. Developing theright technique to encode data, which balances the goals of simplicity, efficiency (in terms of CPUperformance and on disk storage), resilience to corruption, etc., is much of the art of computer science.

As one example, consider the picture taken by a digital camera mentioned above. One encodingtechnique would divide the picture into pixels (dots), and for each pixel, record three bytes ofinformation: the pixel’s "redness", "greenness", and "blueness", each on a scale of 0 to 255. The firstthree bytes of the file would record the information for the first pixel, the second three bytes the secondpixel, and so on. A picture format known as "PNM" does just this (plus some header information, such ashow many pixels are in a row). Many other encoding techniques for images exist, some just as simple,many much more complex.

Text EncodingPerhaps the most common type of data which computers are asked to store is text. As computers havedeveloped, a variety of techniques for encoding text have been developed, from the simple in concept(which could encode only the Latin alphabet used in Western languages) to complicated but powerfultechniques that attempt to encode all forms of human written communication, even attempting to includehistorical languages such as Egyptian hieroglyphics. The following sections discuss many of theencoding techniques commonly used in Red Hat Enterprise Linux.

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ASCIIOne of the oldest, and still most commonly used techniques for encoding text is called ASCII encoding.ASCII encoding simply takes the 26 lowercase and 26 uppercase letters which compose the Latinalphabet, 10 digits, and common English punctuation characters (those found on a keyboard), and mapsthem to an integer between 0 and 255, as outlined in the following table.

Table 2-1. ASCII Encoding of Printable Characters

Integer Range Character33-47 Punctuation: !"#$%&;*(*+,-./48-57 The digits 0 through 958-64 Punctuation: :;<=?>@

65-90 Capital letters A through Z91-96 Punctuation: [\]^_‘

97-122 Lowercase letters a through z123-126 Punctuation: {|}~

What about the integers 0 - 32? These integers are mapped to special keys on early teletypes, many ofwhich have to do with manipulating the spacing on the page being typed on. The following characters arecommonly called "whitespace" characters.

Table 2-2. ASCII Encoding of Whitespace Characters

Integer Character Common Name CommonRepresentation

8 BS Backspace ’\b’

9 HT Tab ’\t’

10 LF Line Feed ’\n’

12 FF Form Feed ’\f’

13 CR Carriage Return ’\r’

32 SPACE Space Bar127 DEL Delete

Others of the first 32 integers are mapped to keys which did not directly influence the "printed page", butinstead sent "out of band" control signals between two teletypes. Many of these control signals havespecial interpretations within Linux (and Unix).

Table 2-3. ASCII Encoding of Control Signals

Integer Character Common Name CommonRepresentation

4 EOT End of Transmission7 BEL Audible Terminal Bell ’\a’

27 ESC Escape

rha030-3.0-0-en-2005-08-17T07:23:17-0400

Copyright (c) 2003-2005 Red Hat, Inc. All rights reserved. For use only by a student enrolled in a Red Hat Academy course taught at a Red Hat Academy.Any other use is a violation of U.S. and international copyrights. No part of this publication may be photocopied, duplicated, stored in a retrieval system, orotherwise duplicated whether in electronic or print format without prior written consent of Red Hat, Inc. If you believe Red Hat course materials are beingused, copied, or otherwise improperly distributed please email [email protected] or phone toll-free (USA) +1 866 626 2994 or +1 (919) 754 3700.

18


Generating Control Characters from the Keyboard: Control and whitespace characters can begenerated from the terminal keyboard directly using the CTRL key. For example, an audible bell canbe generated using CTRL-G, while a backspace can be sent using CTRL-H, and we have alreadymentioned that CTRL-D is used to generate an "End of File" (or "End of Transmission"). Can youdetermine how the whitespace and control characters are mapped to the various CTRL keycombinations? For example, what CTRL key combination generates a tab? What does CTRL-Jgenerate? As you explore various control sequences, remember that the reset command will restoreyour terminal to sane behavior, if necessary.

What about the values 128-255? ASCII encoding does not use them. The ASCII standard only definesthe first 128 values of a byte, leaving the remaining 128 values to be defined by other schemes.

ISO 8859 and Other Character SetsOther standard encoding schemes have been developed, which map various glyphs (such as the symbolfor the Yen and Euro), diacritical marks found in many European languages, and non Latin alphabets tothe latter 128 values of a byte which the ASCII standard leaves undefined. The following table lists a fewof these standard encoding schemes, which are referred to as character sets. The following table listssome character sets which are supported in Linux, including their informal name, formal name, and abrief description.

Table 2-4. Some ISO 8859 Character Sets supported in Linux

Informal Name Formal Name DescriptionLatin-1 ISO 8859-1 West European languagesLatin-2 ISO 8859-2 Central and East European

languagesArabic ISO 8859-6 Latin/ArabicGreek ISO 8859-7 Latin/GreekLatin-9 ISO 8859-15 West European languages

All of these character encoding schemes use a common technique. They preserve the first 128 values of abyte to encode traditional ASCII, and use the remaining 128 values to encode glyphs unique to theparticular encoding. For example, ISO 8859-1 (Latin-1) uses the value 196 to encode a Latin capital Awith an umlaut (Ä), while ISO-8859-7 (Greek) uses the value 196 to encode the Greek capital letterDelta (∆), but both use the value 101 to encode a Latin lowercase e.

Notice a couple of implications about ISO 8859 encoding.

1. Each of the alternate encodings map a single glyph to a single byte, so that the number of lettersencoded in a file equals the number of bytes which are required to encode them.

2. Choosing a particular character set extends the range of characters that can be encoded, but youcannot encode characters from different character sets simultaneously. For example, you could notencode both a Latin capital A with a grave and a Greek letter Delta simultaneously.

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Unicode (UCS)In order to overcome the limitations of ASCII and ISO 8859 based encoding techniques, a UniversalCharacter Set has been developed, commonly referred to as UCS, or Unicode. The Unicode standardacknowledges the fact that one byte of information, with its ability to encode 256 different values, issimply not enough to encode the variety of glyphs found in human communication. Instead, the Unicodestandard uses 4 bytes to encode each character. Think of 4 bytes as 32 light switches. If we were to againlabel each permutation of on and off for 32 switches with integers, the mathematician would tell you thatyou would need 4,294,967,296 (over 4 billion) integers. Thus, Unicode can encode over 4 billion glyphs(nearly enough for every person on the earth to have their own unique glyph; the user prince wouldapprove).

What are some of the features and drawbacks of Unicode encoding?

Scale

The Unicode standard will easily be able to encode the variety of glyphs used in humancommunication for a long time to come.

Simplicity

The Unicode standard does have the simplicity of a sledgehammer. The number of bytes required toencode a set of characters is simply the number of characters multiplied by 4.

Waste

While the Unicode standard is simple in concept, it is also very wasteful. The ability to encode 4billion glyphs is nice, but in reality, much of the communication that occurs today uses less than afew hundred glyphs. Of the 32 bits (light switches) used to encode each character, the first 20 or sowould always be "off".

ASCII Non-compatibility

For better or for worse, a huge amount of existing data is already ASCII encoded. In order to convertfully to Unicode, that data, and the programs that expect to read it, would have to be converted.

The Unicode standard is an effective standard in principle, but in many respects it is ahead of its time,and perhaps forever will be. In practice, other techniques have been developed which attempt to preservethe scale and versatility of Unicode, while minimizing waste and maintaining ASCII compatibility. Whatmust be sacrificed? Simplicity.

Unicode Transformation Format (UTF-8)UTF-8 encoding attempts to balance the flexibility of Unicode, and the practicality and pervasiveness ofASCII, with a significant sacrifice: variable length encoding. With variable length encoding, eachcharacter is no longer encoded using simply 1 byte, or simply 4 bytes. Instead, the traditional 127 ASCIIcharacters are encoded using 1 byte (and, in fact, are identical to the existing ASCII standard). The nextmost commonly used 2000 or so characters are encoded using two bytes. The next 63000 or socharacters are encoded using three bytes, and the more esoteric characters may be encoded using fromfour to six bytes. Details of the encoding technique can be found in the utf-8(7) man page. With full


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backwards compatibility to ASCII, and the same functional range of pure Unicode, what is there to lose?ISO 8859 (and similar) character set compatibility.

UTF-8 attempts to bridge the gap between ASCII, which can be viewed as the primitive days of textencoding, and Unicode, which can be viewed as the utopia to aspire toward. Unfortunately, the"intermediate" methods, the ISO 8859 and other alternate character sets, are as incompatible with UTF-8as they are with each other.

Additionally, the simple relationship between the number of characters that are being stored and theamount of space (measured in bytes) it takes to store them is lost. How much space will it take to store879 printed characters? If they are pure ASCII, the answer is 879. If they are Greek or Cyrillic, theanswer is closer to twice that much.

Text Encoding and the Open Source CommunityIn the traditional development of operating systems, decisions such as what type of character encoding touse can be made centrally, with the possible disadvantage that the decision is wrong for some communityof the operating system’s users. In contrast, in the open source development model, these types ofdecisions are generally made by individuals and small groups of contributers. The advantages of the opensource model are a flexible system which can accommodate a wide variety of encoding formats. Thedisadvantage is that users must often be educated and made aware of the issues involved with characterencoding, because some parts of the assembled system use one technique while others parts use another.The library of man pages is an excellent example.

When contributors to the open source community are faced with decisions involving potentiallyincompatible formats, they generally balance local needs with an appreciation for adhering to widelyaccepted standards where appropriate. The UTF-8 encoding format seems to be evolving as an acceptedstandard, and in recent releases has become the default for Red Hat Enterprise Linux.

The following paragraph, extracted from the utf-8(7) man page, says it well:

It can be hoped that in the foreseeable future, UTF-8 will replaceASCII and ISO 8859 at all levels as the common character encoding onPOSIX systems, leading to a significantly richer environment for han-dling plain text.

Internationalization (i18n)As this Workbook continues to discuss many tools and techniques for searching, sorting, andmanipulating text, the topic of internationalization cannot be avoided. In the open source community,internationalization is often abbreviated as i18n, a shorthand for saying "i-n with 18 letters in between".Applications which have been internationalized take into account different languages. In the Linux (andUnix) community, most applications look for the LANG environment variable to determine whichlanguage to use.

At the simplest, this implies that programs will emit messages in the user’s native language.

[elvis@station elvis]$ echo $LANG


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en_US.UTF-8[elvis@station elvis]$ chmod 666 /etc/passwdchmod: changing permissions of ‘/etc/passwd’: Operation not permitted[elvis@station elvis]$ export LANG=de_DE.utf8[elvis@station elvis]$ chmod 666 /etc/passwdchmod: Beim Setzen der Zugriffsrechte für »/etc/passwd«: Die Operation ist nicht erlaubt

More subtly, the choice of a particular language has implications for sorting orders, numeric formats, textencoding, and other issues.

The LANG environment variableThe LANG environment variable is used to define a user’s language, and possibly the default encodingtechnique as well. The variable is expected to be set to a string using the following syntax:

LL_CC.enc

The variable context consists of the following three components.

Table 2-5. Components of LANG environment variable

Component RoleLL Two letter ISO 639 Language CodeCC (Optional) Two letter ISO 3166 Country Codeenc (Optional) Character Encoding Code Set

The locale command can be used to examine your current configuration (as can echo $LANG), whilelocale -a will list all settings currently supported by your system. The extent of the support for any givenlanguage will vary.

The following tables list some selected language codes, country codes, and code set specifications.

Table 2-6. Selected ISO 639 Language Codes

Code Languagede Germanel Greeken Englishes Spanishfr Frenchja Japanesezh Chinese

Table 2-7. Selected ISO 3166 Country Codes

Code Country





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Code CountryCA CanadaCN ChinaDE GermanyES SpainFR FranceGB Britain (UK)GR GreeceJP JapanNG NigeriaUS United States

Table 2-8. Selected Character Encoding Code Sets

Code Countryutf8 UTF-8iso88591 ISO 8859-1 (Latin 1)iso885915 ISO 8859-15 (Latin 10)iso88596 ISO 8859-6 (Arabic)iso88592 ISO 8859-2 (Latin 2)

See the gettext info pages (info gettext, or pinfo gettext) for a complete listing.

Do I Really Have to Know All of This?We have tried to introduce the major concepts and components which affect how text is encoded andstored within Linux. After reading about character sets and language codes, one might be led to wonder,do I really need to know about all of this? If you are using simple text, restricted to the Latin alphabet of26 characters, the answer is no. If you are asking the question 10 years from now, the answer willhopefully be no. If you do not fit into one of these two categories, however, you should have at least anacquaintance with the concept of internationalization, character sets, and the role of the LANGenvironment variable.

Hopefully, as the open source community converges on a single encoding technique (currently UTF-8seems the most likely), most of these issues will disappear. Until then, these are some key points toremember.

1. An ASCII file is already valid in one of the ISO 8559 character sets.

2. An ASCII file is already valid in UTF-8.

3. A file encoded in one of the ISO 8559 character sets is not valid in UTF-8, and must be converted.

4. Using UTF-8, There is a one to one mapping between characters and bytes if and only if all of thecharacters are pure ASCII characters.


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If you are interested in more information, several man pages provide a more detailed introduction to theconcepts outlined above. Start with charsets(7), and then follow with ascii(7), iso_8859_1(7),unicode(7) and utf-8(7). Additionally, the iconv command can be used to convert text files from oneform of encoding to another.

Notes1. While this may seem an obvious way to do things, some operating systems take more elaborate

approaches. The Macintosh operating system, for example, stores file using two arrays ofinformation, a data fork and a resource fork.


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Chapter 3. The RPM Package Manager

Key Concepts• The rpm command is used to add or remove software from your system.

• You must have root privileges to add and remove software with rpm.

• Anyone can query installed packages, or package files.

Discussion

RPM: The Red Hat Package ManagerThe Red Hat Package Manager is probably the element that most defines the Red Hat Enterprise Linuxdistribution. The package manager allows developers a way to build and distribute software,administrators a way to install and maintain software, and all users a way to query for information aboutand verify the integrity of installed software.

The Need for Package ManagementBefore package management became a mainstream concept, software was primarily distributed as"tarballs" (i.e., compressed tar archives), and often only in source form. Anyone using an open sourceproduct would need to unpack the tarball, compile the executables (hoping their system had the rightlibraries to support it), and install the product on their system, often with components in multipledirectories (such as /etc, /var/lib, /usr/bin/, and /usr/lib).

Usually, scripts which are distributed with open source software make each of the above steps easier, buteven they do not address two fundamental problems. The first is the dependency problem. Often, opensource products reuse code which is distributed in the form of a library. If the appropriate library is notalready installed on the system, the application which depends on it will be useless. The secondfundamental problem is maintenance. What happens when, six months down the road, a new version ofthe product is released, or someone decides they do not want a particular product anymore? Theindividual files of the product would need to be tracked down and removed or replaced.

RPM is designed to help resolve both of these fundamental problems.

RPM ComponentsWhen people speak of RPM, they are speaking of three components collectively: the RPM database, therpm executable, and package files.

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The RPM databaseThe RPM database is the heart of the product, and provides the answer for both problems stated above.Whenever software is installed by RPM, database entries representing every file installed are created.Because of the database, the files can be easily removed from the system at a later time.

Additionally, the database maintains a list of dependencies required and provided by various packages.When installing an application which requires a library, for example, the library can be listed as a specificdependency, and the database can unambiguously vouch for the presence (or absence) of the library.

The RPM database resides in the directory /var/lib/rpm, but other than knowing that it exists, there islittle need for standard users (or even administrator) to access the directory directly.

The rpm ExecutableUsers interact with the RPM database through the front end rpm command. The rpm executable is usedby administrators to install, upgrade, or remove software packages, and by any user to answer questionsabout installed packages or verify their integrity. We will discuss RPM queries in detail below.

Package FilesPackage files are the means by which software is distributed. Packages file are generally named using thefollowing convention.

name-version-release.arch.rpm

For example, Red Hat’s first release of the package file for version 4.0.7 of the open source applicationzsh compiled for the Intel x86 (and compatible) architecture would conventionally be namedzsh-4.0.7-1.i386.rpm.

Package files are essentially tar archives (though they more closely resemble less familiar cpio archives)combined with header information which names, versions, and states dependencies for the package.When people refer to the Red Hat distribution, they are generally referring to the collecting of RPMpackage files which compose the software installed on a machine.

Querying the RPM databaseWhen invoked with -q as its first command line switch, the rpm command will perform queries againstthe RPM database (or sometimes RPM package files directly).

Formulating RPM queriesWhen first introduced to rpm, the syntax associated with queries can be a little overwhelming. It helps tothink of every query as being composed of two questions: (1) What packages am I querying? (2) Whatquestion am I asking? Each of the rpm command’s query related switches will fall into one of the twocategories.





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Table 3-1. RPM Query Options for Specifying Packages

Option Specification-a all installed packagespackage-name the package name-f filename the package that owns the file filename-ppackage-file-name

query the package file package-file-name directly. This option isfundamentally different, as all other options query the RPM database ofinstalled packages.

Table 3-2. RPM Query Options for Specifying Information

Option Specification(default) package name and version-i package information header-l list of files owned by the package--queryformat str list information specified in format string str

By choosing one option from the first table, and zero or more options from the second table, users canformulate specific questions for the RPM database.

Query Examples

General QueriesFor example, the -a command line switch performs a query against all installed packages. If no otherquestion is asked, by default rpm returns the package name. Thus rpm -qa will return a list of allinstalled packages.

[prince@station prince]$ rpm -qabasesystem-8.0-2expat-1.95.5-2libacl-2.2.3-1popt-1.8-0.69rootfiles-7.2-6cpio-2.5-3gzip-1.3.3-9...

If a package name is specified, the rpm will query only that package. What information is returned?Again, by default, the package name.

[prince@station prince]$ rpm -q bashbash-2.05b-20.1

rha030-3.0-0-en-2005-08-17T07:23:17-0400

Copyright (c) 2003-2005 Red Hat, Inc. All rights reserved. For use only by a student enrolled in a Red Hat Academy course taught at a Red Hat Academy. Any other use isa violation of U.S. and international copyrights. No part of this publication may be photocopied, duplicated, stored in a retrieval system, or otherwise duplicated whetherin electronic or print format without prior written consent of Red Hat, Inc. If you believe Red Hat course materials are being used, copied, or otherwise improperly distributedplease email [email protected] or phone toll-free (USA) +1 866 626 2994 or +1 (919) 754 3700.

27


While perhaps not the most informative of queries, prince at least receives confirmation that the packageis installed, and a version number. Usually, when querying a package name, more information isrequested. For example, adding -i will generate an information header.

[prince@station prince]$ rpm -qi bashName : bash Relocations: /usrVersion : 2.05b Vendor: Red Hat, Inc.Release : 20.1 Build Date: Wed 09 Apr 2003 09:02:36 AM EDTInstall Date: Tue 08 Jul 2003 09:29:33 AM EDT Build Host: stripples.devel.redhat.comGroup : System Environment/Shells Source RPM: bash-2.05b-20.1.src.rpmSize : 1619204 License: GPLSignature : DSA/SHA1, Mon 09 Jun 2003 06:45:19 PM EDT, Key ID 219180cddb42a60ePackager : Red Hat, Inc. <http://bugzilla.redhat.com/bugzilla>Summary : The GNU Bourne Again shell (bash).Description :The GNU project Bourne Again shell (bash) is a shell or commandlanguage interpreter that is compatible with the Bourne shell(sh). Bash incorporates useful features from the Korn shell (ksh) andthe C shell (csh) and most sh scripts can be run by bash withoutmodification. Bash is the default shell for Red Hat Linux.

Or by adding a -l, a list of all installed files is generated.

[prince@station prince]$ rpm -ql bash/bin/bash/bin/bash2/bin/sh/etc/skel/.bash_logout/etc/skel/.bash_profile/etc/skel/.bashrc/usr/bin/bashbug/usr/lib/bash/usr/share/doc/bash-2.05b/usr/share/doc/bash-2.05b/CHANGES...

Including both, as in rpm -qil bash, would generate both.

Investigating an Unfamiliar PackageWhat if you have come across a file in the filesystem, and want to know to which package the filebelongs? rpm -qf ...

[prince@station etc]$ rpm -qf /etc/aep.confhwcrypto-1.0-14

Want to know more about the package? Add a -i.

[prince@station etc]$ rpm -qf /etc/aep.conf -iName : hwcrypto Relocations: (not relocateable)Version : 1.0 Vendor: Red Hat, Inc.Release : 14 Build Date: Tue 04 Feb 2003 06:20:37 AM ESTInstall Date: Tue 01 Apr 2003 11:27:43 AM EST Build Host: sylvester.devel.redhat.com


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Group : System Environment/Base Source RPM: hwcrypto-1.0-14.src.rpmSize : 711506 License: GPLSignature : DSA/SHA1, Mon 24 Feb 2003 01:25:46 AM EST, Key ID 219180cddb42a60ePackager : Red Hat, Inc. <http://bugzilla.redhat.com/bugzilla>Summary : Hardware cryptographic accelerator support.Description :This package contains the shared libraries used to interface withhardware cryptographic accelerators under Linux.

Is there any documentation on the system that could tell you more about it? Add a -l to list the filesrelated to /etc/aep.conf.

[prince@station etc]$ rpm -qf /etc/aep.conf -l/etc/aep/etc/aep.conf/etc/aep/aeptarg.bin/etc/aeplog.conf.../usr/sbin/aepversion/usr/share/doc/hwcrypto-1.0/usr/share/doc/hwcrypto-1.0/hwcrypto.txt/usr/share/doc/hwcrypto-1.0/readme.snmp/usr/share/snmp/mibs/cnStatTrap.mib

In this case, not much, but maybe /usr/share/doc/hwcyrpto.txt will provide some help. Manypackages include man pages that can be read, or info pages that can be browsed. At least you can locatesome configuration files you might want to peruse to find out more.

Investigating an Unfamiliar Package FileWhat if you come across a package file which is not yet installed on your system? The rpm commandallows package files to be queried directly with the -p command line switch.

[prince@station RPMS]$ rpm -qil -p xsri-2.1.0-5.i386.rpmName : xsri Relocations: (not relocateable)Version : 2.1.0 Vendor: Red Hat, Inc.Release : 5 Build Date: Sat 25 Jan 2003 03:37:15 AM ESTInstall Date: (not installed) Build Host: porky.devel.redhat.comGroup : Amusements/Graphics Source RPM: xsri-2.1.0-5.src.rpmSize : 27190 License: GPLSignature : DSA/SHA1, Mon 24 Feb 2003 12:40:17 AM EST, Key ID 219180cddb42a60ePackager : Red Hat, Inc. <http://bugzilla.redhat.com/bugzilla>Summary : A program for displaying images on the background for X.Description :The xsri program allows the display of text, patterns, and images inthe root window, so users can customize the XDM style login screenand/or the normal X background.

Install xsri if you would like to change the look of your X loginscreen and/or X background. It is also used to display the defaultbackground (Red Hat logo)./usr/bin/xsri/usr/share/doc/xsri-2.1.0


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/usr/share/doc/xsri-2.1.0/README

As mentioned in the table above, this is a fundamentally different type of query. The package file, whichmight or might not be installed, is providing the information, not the RPM database.

Formatting Specific InformationWhat if you would like to generate a list of the 10 largest packages installed on your system? With rpm-qai, the information header for every package would be displayed, which could be grepped down forjust the sizes, but then you’d need names. You could add names, but then the name and size would be onseparate lines. You get the idea.

Fortunately, the rpm command allows users to compose very specific questions by specifying a queryformat string. The string is composed of any ASCII text, but tokens of the form %{fieldname} will bereplaced with relevant information field. What can be used as filed names? For starters, any field found ina package’s information header, but there’s more. The command rpm --querytags will return a complete(and intimidating) list of available fields.

For the specific task at hand, prince performs the following query. (Note he needs to explicitly specify anewline with \n).

[prince@station RPMS]$ rpm -qa --queryformat "%{size} %{name}\n"0 basesystem156498 expat19248 libacl111647 popt1966 rootfiles67679 cpio162449 gzip...

Just the information prince wanted. With a syntax of %width{fieldname}, an optional field width can bespecified. Using this to clean up his output, and piping to sort and head, prince generates a list of the 10largest packages on his system fairly easily.

[prince@station RPMS]$ rpm -qa --queryformat "%10{size} %{name}\n" | sort -rn |head170890527 kernel-source131431309 openoffice-i18n100436356 openoffice-libs84371104 gnucash80018678 openoffice75838208 rpmdb-redhat55166532 Omni54674111 tetex-doc41939971 glibc-common36762653 xorg

rha030-3.0-0-en-2005-08-17T07:23:17-0400


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Online Exercises

Lab ExerciseObjective: Become familiar with RPM queries

Estimated Time: 15 mins.

Specification

1. Create the file ~/bash_files, which contains a list of all files which belong to the bash package,listing one file per line using absolute references.

2. Create the file ~/sshd_man, which lists the three files which contain man pages associated with theopenssh-server package, one file per line using absolute references.

3. In the file ~/whatis_libcap, include the single word which best completes the followingsentence: The /lib/libcap.so.1.* library is used for getting and setting POSIX.1e __________.(Do not be concerned if you do not fully understand the answer).

4. Create the file ~/license_counts, which tables the number of occurrences of packages which arelicensed under a given license, for the top 5 most commonly used licenses, sorted in numericallydescending order. If performed correctly, your file should be formatted similarly to the following.(Do not be concerned if the actual counts or license names are different. Also, you might noticelogically similar licenses, such as LGPL/GPL and GPL/LGPL. Do not make any attempt to combinethem into a single entry.)[prince@station prince]$ cat license_counts

355 GPL147 LGPL53 BSD47 distributable18 xorg

Deliverables

1. The file ~/bash_files, which contains a list of all files owned by the bash package, one file per line, usingabsolute references.

2. The file ~/sshd_man, which contains a list of the three files which provide man pages for the openssh-serverpackage, one file per line, using absolute references.

3. The file ~/whatis_libcap, which contains the one word answer for what the library gets and sets.

4. The file ~/license_counts, which tables the various licenses under which packages are distributed,


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proceeded by the number of packages to which the license applies, sorted in numerically descending order.

Questions

1. How does almost every RPM query command line begin?

( ) a. rpmquery ...

( ) b. rpm -q ...

( ) c. qpackage ...

( ) d. lsrpm ...


2. Where is the RPM database located?

( ) a. /tmp/.rpmdb( ) b. /usr/share/rpm( ) c. At http://rpmdb.redhat.com

( ) d. /var/lib/rpm( ) e. None of the above

3. What would be the conventional name of the package file for release 7 of version 2.0.8 of the bash packagecompiled for the x86 architecture?

( ) a. bash.i386-2.0.8.7.rpm( ) b. rpm-bash-2.0.8-7.i386( ) c. bash-2.0.8-7.i386.rpm( ) d. bash-2.0.8-i386.rpm( ) e. None of the above

4. Which of the following command lines would list the package names for all installed packages?

( ) a. rpm -q --dump

( ) b. rpm -qa

( ) c. lsrpm -a

( ) d. rpm -q --name



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5. Which of the following would generate an information header and file list for (only) the package xsnow?

( ) a. rpm -q --list -l xsnow

( ) b. rpm -qa -i -l

( ) c. rpm -q -i xsnow

( ) d. rpm -qil xsnow


6. Which of the following command lines would list all files which are contained in the same package as/etc/pwdb.conf?

( ) a. rpm -ql /etc/pwdb.conf

( ) b. rpm -fql /etc/pwdb.conf

( ) c. rpm -qlf /etc/pwdb.conf

( ) d. rpm -qif /etc/pwdb.conf


7. Which of the following command lines would query the xsane-0.89-3.i386.rpm package file for a list of filesthat it contains?

( ) a. rpm -q -p xsane-0.89-3.i386.rpm -l

( ) b. rpm -ql xsane-0.89-3.i386.rpm

( ) c. rpm -qp xsane-0.89-3.i386.rpm

( ) d. rpm -qip xsane-0.89-3.i386.rpm


8. Which of the following could be used to determine how much disk space the xscreensaver package consumes?

( ) a. rpm -q -i xscreensaver

( ) b. rpm -q -s xscreensaver

( ) c. rpm -qa xscreensaver

( ) d. rpm -q -l xscreensaver



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9. Which of the following commands could be used to determine which version of the up2date package is installed?

( ) a. rpm -q up2date

( ) b. rpm -qi up2date

( ) c. rpm -q --queryformat "%{Version}\n" up2date

( ) d. B and C Only

( ) e. A, B, and C

10. Which of the following would generate a table of installed package sizes and names for all packages files in thelocal directory?

( ) a. rpm --queryformat "%10{Size} %{Name}\n" -p *.rpm

( ) b. rpm -q --queryformat "%10{Size} %{Name}\n" *.rpm

( ) c. rpm -q --queryformat "%10{Size} %{Name}\n" -p *.rpm

( ) d. rpm -qf "%10{Size} %{Name}\n" -p *.rpm


rha030-3.0-0-en-2005-08-17T07:23:17-0400


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